I've not done it, but I'm sure it's no small task. You may even lose your backseat in the process. You certainly will have to modify the trunk area. The whole thing seems rediculously laborious just to fit 10's under the car. You could go with 7's and a 225 series tire without modification. Sounds like you've got a lot to do before you get to wheels and tires. *G*
They may fit on there...I don't know. The standard modifications for installing wheels/tires that are larger than can possibly fit with stock arrangements are narrowing the rear end, "mini tubs" which are basically custom inner fender wells, inboard movement of the leaf springs, and fender flares. All are a lot of trouble and require extensive modification to your Mustang. You can probably fit up to about a 9" or so in the back...so I don't know if the effort would be worth it for just another inch or so. I don't really have any experience with '67's, but there is a max fitment chart on the Total Control Products website that may help you a bit. Remember that if you are going to be using this to go around corners you have to leave more clearance between the tire and the leafspring than if you were just going straight.